Arc fault circuit breakers or AFCI’s have been required by the national electrical code starting in 2002, for all bedroom circuits and then in 2005 they change the code to incorporate all circuits in a residence that are not GFCI (Ground Fault Circuit Interrupters) protected or dedicated major appliance outlets.
Similar to a GFCI the AFCI will interrupt power to a circuit when it senses a fault or arcing to prevent an arc from becoming a fire. They are still rated in common amperages of 15 and 20 amp and fit your breaker panel the same as a standard breaker. In some brands such as Square D the physical size of the breaker is the same width but twice as long so it is a little more difficult to fit into the panel, other brands are basically the same size as a standard breaker. All the brands have a few things in common. They all have a white pigtail that must be connected to the neutral bar of the panel and the neutral coming from the branch circuit is connected to the breaker on a designated terminal in the same manner as a GFCI breaker. They also have a color coded button or trip indicator on the front by the handle to let you know what type of breaker they are.
The introduction of the AFCI breaker has changed how you may be used to wiring a home because they will not work if the circuit has a shared neutral or part of a (balanced set). When I started wiring homes we used to use a 3 conductor romex for all of the homerun wires where you have 2 hot wires and one shared neutral. Now every home run has to be a 2 conductor cable so each circuit has its own neutral. This just adds a few more cables to the panel.
Because the AFCI’s require their own neutral it can be difficult to install them in an older home say pre 1970. In the older homes it was common practice to use whatever neutral was closest to the light or switch even though it was not from the same home run cable. It is possible to go through and trace all that out but it takes time and rooting around in the attic through the old insulation that makes you cough and itch.
Even though the new technology is a little more difficult for the electrician and more expensive for the consumer I believe they are a great step towards preventing electrical fires. If a varmint chews through a wire inside the wall or you happen to nick a wire with a nail trying to hang a picture the breaker will trip before a major problem can occur.